The longest formula is, his probably isn't the longest equation, but the Lagrangian for the standard model (which describes all observed physical processes, sans gravity, given locality, causality, Lorentz invariance, and known physical data since 1860) is moderately lengthy:
The longest formula is, his probably isn't the longest equation, but the Lagrangian for the standard model (which describes all observed physical processes, sans gravity, given locality, causality, Lorentz invariance, and known physical data since 1860) is moderately lengthy: